Third generation (3G) mobile communication systems (e.g. Universal Mobile Telecommunications System (UMTS)) shall offer high quality voice and data services for mobile users. The systems shall also provide high capacity and universal coverage. In some situations that may however be difficult to fulfil, due to unreliable radio channels. One promising technique to combat link reliability problems over the radio interface is macro diversity techniques. Macro diversity should however also be seen as an inherent consequence of using Code Division Multiple Access (CDMA) as the multiple access technique in a cellular network. CDMA is an interference limited technology. That is, it is the interference in a cell that sets the upper limit for the cell's capacity. To keep the interference as low as possible it is essential that the base station controls the output power of the radio transmitters of the mobile terminals in the cell, i.e. fast and efficient power control is essential. As a mobile terminal moves towards the periphery of a cell it has to increase the power of its radio transmission in order for the base station to be able to receive the transmitted signal. Likewise, the base station has to increase the power of its radio transmission towards the mobile terminal. This power increase has a deteriorating effect on the capacity of both the mobile terminal's own cell and the neighboring cell(s) which the mobile terminal is close to. Macro diversity is used to mitigate this effect. When the mobile terminal communicates via more than one base station, the quality of the communication can be maintained with a lower radio transmission power than when only a single base station is used. Thus, macro diversity is both a feature raising the quality of unreliable radio channels and a necessity that is required in order to overcome an inherent weakness of CDMA based cellular systems.
The present invention relates to arrangements in a radio access network of a cellular mobile network comprising means for handling macro diversity functions. An example of such a radio access network is the UMTS terrestrial radio access network (UTRAN). The UTRAN is illustrated in FIG. 1 and comprises at least one Radio Network System (RNS) 100 connected to the Core Network (CN) 200. The CN is connectable to other networks such as the Internet, other mobile networks e.g. GSM systems and fixed telephony networks. The RNS 100 comprises at least one Radio Network Controller (RNC) 110. Furthermore, the respective RNC 110 controls a plurality of Node-Bs 120, 130 that are connected to the RNC by means of the Iub interface 140. Each Node B covers one or more cells and is arranged to serve the User Equipment (UE) 300 within said cell(s). Finally, the UE 300, also referred to as mobile terminal, is connected to one or more Node Bs over the Wideband Code Division Multiple Access (WCDMA) based radio interface 150. The network of FIG. 1 is also referred to as a WCDMA network and is based on the WCDMA standard specified by the 3:rd Generation Partnership Project (3GPP).
Macro diversity implies in this description that the mobile terminal communicates with more than one base station simultaneously, i.e. the same data flow is transmitted to/from the mobile terminal from/to at least two different base station. When two or more base stations receive/transmit the same data flow, the data flows are combined/split at a higher node, referred to as a diversity handover node (DHO node). The DHO node may be a RNC, or a Node B (or another type of node such as a specialized DHO node or a new type of control node) when a distributed macro diversity scheme is used as explained below.
In current UTRANs macro diversity is realized through macro diversity functionality (i.e. DHO functionality) in the RNCs. The standard allows DHO functionality in both the serving RNC (SRNC) and the drift RNC (DRNC), but many vendors have chosen not to use the possibility to locate DHO functionality in the DRNC. The standard also allows up to six macro diversity legs, i.e. six Node Bs in the active set, but products may be limited to be implemented such that only two and three leg macro diversity is possible, i.e. maximum three Node Bs in the active set. The reason for limiting the active set to three Node Bs is that very little gain is expected from additional legs.
Thus, in existing macro diversity solutions the user data is transported in multiple separate data flows, one for each macro diversity leg, all the way between the RNC and the Node Bs. This consumes costly transmission resources in the UTRAN transport network. A more transmission efficient way to realize the macro diversity in a UTRAN is to distribute the DHO functionality to the Node Bs (and the RNCs). The result is not only distributed macro diversity functionality but also hierarchical macro diversity functionality, since more than two macro diversity legs may result in more than one DHO node. The benefits that can be achieved depend on the topology of the UTRAN transport network. FIGS. 2 and 3 illustrate two cases when the distributed DHO functionality is beneficial compared to having the DHO functionality only in the RNC. It should be noted that the DHO functionality could also be distributed to other types of nodes than Node Bs (base stations) and RNCs, e.g. specialized DHO nodes or new types of control nodes. Having noted this, the further description of the present invention will, for the sake of clarity, focus on RNCs and Node Bs (base stations) as DHO nodes, but the present invention is applicable also when other types of DHO nodes are used.
Mechanisms for such a distributed macro diversity scheme have been described in WO2005/62654, WO2005162655 and WO2005/62495. The scheme, which is controlled by the SRNC, includes:                mechanisms for selection of the Node Bs that are to perform the macro diversity functions (splitting and combining), i.e. the distributed DHO nodes;        mechanisms for directing the data flows between the selected DHO nodes and providing the DHO nodes with the information that they need in order to perform the splitting and combining; and        a timing scheme to handle the trade off between delay and data loss.        
The present invention deals with the first of the above three items, i.e. selection of DHO nodes, i.e. to select which of the DHO enabled nodes that is to perform the macro diversity function. A number of real-time DHO node selection schemes are described in WO2005/62654 and WO2005/62655. The major parts of the invention can be used also together with other DHO node selection schemes than the ones described in WO2005/62654 and WO2005/62655. Common for the DHO node selection schemes is that they rely on topology information from the transport network layer to select the most appropriate DHO nodes. An RNC can retrieve the necessary topology information in four different ways:    1. Through manual or semi-automatic management operations.    2. Via a link state routing protocol.    3. Using a traceroute mechanism that allows the RNC to discover the hop-by-hop route to each Node B in an IP UTRAN.    4. Through signaling an RNC may possibly retrieve topology information from other RNCs to cater for the inter-RNS case (if applicable).
The traceroute method may be the most attractive one, but it only works in IP based UTRANs.
The retrieved topology information is stored in a topology database in the RNC which is updated when/if the topology of the UTRAN changes. The information in the topology database includes (at least):                The hop-by-hop route from the RNC to each Node B in the RNS and possibly to some Node Bs in neighboring RNSs to cater for inter-RNS soft handover cases.        A delay metric for each hop in a route.        Some sort of generic cost metric for each hop in a route. The generic cost metric reflects the operator's relative willingness to use the link for data transport. In the simplest case the delay metric is used as the generic cost metric.        Information about which nodes that are DHO enabled, i.e. the Node Bs comprising macro diversity function. However, this information does not have to be included in the topology database as such. It may alternatively be stored elsewhere in the RNC.        
The goal of a DHO node selection algorithm is to select DHO nodes in a way that minimizes the accumulated generic cost metrics for the all the macro diversity legs put together with the condition that for none of the resulting data paths is the resulting path delay allowed to exceed a maximum delay value defined for the UTRAN.
The above mentioned real-time DHO node selection algorithms use the information in the topology database to create a “route tree” that is the basis for the DHO node selection for a certain macro diversity configuration i.e. for a certain combination of Node Bs for a macro diversity connection. Then a set of preliminary DHO nodes are selected and together with the SRNC, any involved non-DHO node Node Bs and the relative interconnections of the nodes they form a conceptual “DHO node tree”.
Then the RNC checks that the resulting path delay will not exceed the maximum allowed path for any of the macro diversity legs. If a path delay is too large, DHO nodes are removed until the path delay is acceptable for all macro diversity legs. Alternatively the delay checks may be integrated in the selection of the preliminary DHO nodes.
The real-time DHO node selection is executed when needed, i.e. when the macro diversity configuration of a connection is established or changed (i.e. when a Node B/base station is added or removed from the combination of Node Bs/base stations used for a macro diversity connection).
Although the above-mentioned existing DHO node selection schemes work, they do have certain disadvantages. They are all calculation-intensive procedures that are to be executed under a real-time pressure. Therefore, the existing DHO node selection schemes may adversely affect the real-time properties of connection establishment i.e. introduce increased delay and may therefore delay changes in the macro diversity configuration of a connection i.e. additions or removals of Node Bs taking part in the macro diversity connection.
Another aspect of this problem is that the introduced increased delay limits the complexity that can be allowed for a real-time DHO node selection scheme and thus limits the potential accuracy that the scheme can achieve, i.e. how close its DHO node choices are to the theoretically ideal choices.
The problems with the introduced increased delay may be addressed by adding more computation resources in the RNC, but that would instead unnecessarily increase the cost of the node.